Sanctuary Integrated Monitoring Network
Monitoring Project

Nutritional Constraints on Sea Otters in the Monterey Bay National Marine Sanctuary

Principal Investigator(s)

  • Olav Oftedal
    Smithsonian National Zoological Park
  • Katherine Ralls
    Smithsonian National Zoological Park


  • SIMoN
Start Date: June 05, 2004

The slow growth of the threatened sea otter population in California is accompanied by a number of other facets (such as female mortality, reduced pup abundance, lower mass to length ratios, changes in prey utilization, high rate of infectious disease, and prevalence of cardiomyopathy) that are consistent with nutritional inadequacies in otter diets. Unfortunately the potential importance of nutrition to population health of sea otters has largely been overlooked both by managers and by scientists, a shortcoming that could hinder efforts to promote recovery of this species in California. A balanced approach to sea otter conservation requires that nutrition be considered one of the potential constraints on sea otter populations, in addition to accidental drowning in fishing gear, environmental sources of pathogens, potential exposure to toxins and contaminants, reduced genetic variation, and impaired immune function.

We are investigating nutritional constraints on sea otters via examination of the nutrient composition of sea otter prey while coupling these data with studies on otter foraging behavior. By piggybacking this study onto an existing multiyear research effort using radio-telemetered otters in the MBNMS, we will develop the most detailed evaluation of individual variation in diet for any marine mammal and gain an understanding of the impact of diet change on the nutrient intakes of this species. This information is essential for determination of appropriate management decisions to protect or enhance key nutritional resources that may be limiting otter populations.

Our research focuses on four specific objectives:

Objective 1. Examine geographical and seasonal variation in proximate composition, major minerals and vitamin E in the 10 major prey types of sea otters in and near MBNMS.

Objective 2. Measure other vitamins, trace minerals, amino acids and fatty acids in 10 major prey items of sea otters in summer, the season of maximal reproductive stress and adult mortality.

Objective 3. Estimate the nutrient composition of individual otter diets, based on multiyear data on food intake in both northern and southern parts of MBNMS, and correlate these data to age/sex differences, geography, reproductive history, and, where available, data on morbidity and mortality.

Objective 4. Compare nutritional data obtained in Objective 3 (primarily from rocky-bottom habitats in MBNMS) to nutritional estimates for historic diets from soft-sediment habitats (especially Elkhorn Slough) and diets in an expanding otter population in Glacier Bay, SE Alaska.

Summary to Date

Based on data summarized by Dr. Tim Tinker in his 2004 PhD dissertation, the major prey categories for sea otters in central California were redefined, on a mass basis:
1. cancer crabs (21%),
2. clams (16%),
3. abalone (13%),
4. innkeeper worm (8%),
5. purple urchin (8%),
6. mussels (8%),
7. turban snails (7%),
8. kelp crab (7%),
9. other crabs (5%),
10. octopus (4%), and
11. sea stars (3%).

Given that the diets of individual otters fall into differing prey groupings, some prey types (such as clams and snails) are particularly important for individual otters. We revised our list of 10 major prey types that will be sampled on a geographic and seasonal basis.

Large cancer crabs (Cancer magister/ C. antennarius)
Kelp crabs (Pugettia producta)

Mussels (Mytilus californianus)
Large clams (Saxidomus nuttalli/Tresus nuttallii)
Small clams (Protothaca staminea, Macoma spp.)

Abalone (Haliotis rufescens/H. cracherodii)
Turban snails (Tegula funebralis/T. montereyi/T. brunnea/T. pulligo)

Sea stars (Pisaster ochraceus/P. giganteus)
Urchins (Strongylocentrotus purpuratus)

Echiurid worms
Innkeeper worms (Urechis caupo)

In 2004, summer collection activities began on June 5 and continued through June 25, with one collection on July 8. Fall 2004 collections began on September 9 and continued through the month as dive trips could be arranged. About 76 species were collected at sites in the northern and southern end of the Monterey Bay National Marine Sanctuary. Note that some of these collections involved only a few individuals, and may not be sufficient for nutritional analysis if additional individuals are not obtained.

As of Sept. 30 2004, the Nutrition Lab at the Smithsonian National Zoological Park processed (including dry matter determination) 155 samples representing 27 invertebrate species. A subset of these was ether extracted for determination of fat content, and another subset was digested in nitric acid in a microwave digestion system preparatory to mineral analyses but these data have yet to be summarized.


This research involves close collaboration with multiple agencies, individuals and projects to obtain and analyze the prey samples. Samples are processed and most assays conducted at the Nutrition Laboratories at the Smithsonian National Zoological Park. A subset of samples will be sent to reputable commercial laboratories for additional vitamin, trace mineral, amino acid and fatty acid analyses.

Our project will contribute to the goals and concerns of MBNMS/SIMoN in several ways. One important contribution will be the provision of timely and pertinent information to the research community, managers and decision-makers, and the general public. This will be accomplished through our final report to MBNMS, scientific papers, presentations at scientific meetings, popular articles, stories on the Smithsonian and SIMoN web sites, and incorporation of our data into the Wild Sea Otter Database at UC Santa Cruz. Our results will be of considerable interest to our collaborators who are directing the current radiotelemetry studies of sea otter foraging ecology in the sanctuary, as well as researchers (DFG and UC Davis) studying sea otter disease exposure, contaminant levels, and other health parameters in parallel with that study.

Information on the nutritional composition of wild sea otter diets will be highly useful to the sea otter husbandry and rehabilitation community including staff at the Monterey Bay Aquarium and The Marine Mammal Center (in Sausalito), as it can be used to improve dietary formulations and veterinary treatment of captive and live-stranded sea otters. Our findings will assist the US Fish and Wildlife Service with sea otter recovery efforts by assessing the possible contribution of nutritional stress to the current high rate of mortality and morbidity from infectious diseases in the otter population.

Our project will also provide baseline data on the nutritional adequacy of sea otter diets within the MBNMS and for comparison to the diet of a healthy and rapidly growing sea otter population in Alaska. Sea otters are a keystone species in near-shore communities within MBNMS, yet no information exists on the nutritional adequacy of their diets. Thus, our project fills an important gap in the current understanding of the status of marine resources within MBNMS and contribute to SIMoN's mission of identifying and tracking natural and human-induced changes to MBNMS.

Study Parameters

  • Trophic association
  • Behavior
  • Habitat association
  • Reproduction
  • Range/Biogeography
  • Growth
  • Nutrient composition of subtidal inverts
  • Predation
  • Distribution
  • Migration/movement patterns

Study Methods

The 10 major prey types of sea otters will be the primary focus for geographic and seasonal sampling. Individual diets of radio-tagged sea otters are currently being monitored at two general sampling areas: the Monterey Peninsula and southern Monterey Bay (northern part of MBNMS) and the San Simeon area (southern MBNMS and beyond). We will collect within the feeding ranges of tagged sea otters at these two study areas (e.g., either kelp beds, open rocky sub-tidal, sand-dominated sub-tidal, rocky intertidal, or intertidal mud flats, as appropriate to each prey species). To examine seasonal variation, we will sample four times during the year: A. Dec-Jan; B. Mar-April ; C. May-July; D. Aug-Oct. To allow statistical comparisons by analysis of variance, replicate samples will be obtained for each prey type for each season and site. Where feasible, we will also collect a few species separately by sex to account for differences attributable to gonadal development, even though we do not know whether otters distinguish between sexes of prey.

Inter-tidal sample collection will be conducted by shore-based field workers during low tide series, while sub-tidal sample collection will be conducted by certified scientific divers based out of small boats. Researchers and volunteers from a number of institutions and agencies – UCSC, UC Davis, USGS, California Dep. Fish & Game (CDFG), the Monterey Bay Aquarium (MBA) and the Smithsonian Institution – will participate in the intertidal collections. For the sub-tidal collections, dive teams may include divers from UCSC, USGS, CDFG, UC Santa Barbara, and the Smithsonian Institution.

Each sample needs to contain sufficient edible portion for analysis. Each sample will contain at least 2 individual prey items, if large, or many more, if small. The maximum length and mass of individual prey will be measured to develop size: mass relationships. In some major prey types, we will compare prey of more than one size class, in case size has an effect on nutrient composition independent of sexual maturity.

All samples will be stored frozen until shipped to the Smithsonian National Zoological Park’s Nutrition Laboratory for processing and analysis. The prey items will be manually separated into portions that otters normally eat and those that they normally do not eat. Both portions will be freeze-dried to determine fresh and dry weights. The edible portion will be ground or homogenized in a Wiley food mill, Spex mill or homogenizer, depending on texture, rigidity and fat content, and subsampled for nutritional analysis. Subsamples will be assayed at the SNZP nutrition lab for residual water content by oven drying at 100 C, for nitrogen content by CHN elemental gas analysis and/or Kjeldahl digestion procedures, for fat by ether extraction in Soxhlet extractors, and for gross energy by combustion in an adiabatic bomb calorimeter. Subsamples to be assayed for mineral constituents will be wet-ashed in nitric acid in a high-pressure microwave digestion system. Major minerals such as calcium, potassium and magnesium will be analyzed by flame atomic absorption spectroscopy, but phosphorus will be assayed by spectrophotometry using the Gomorri method. Some trace mineral analyses will be conducted by flame or furnace atomic absorption techniques, depending on concentration levels. Assays will be validated and checked against known standard reference materials.

A subset of the prey samples obtained during the summer season will be sent to commercial labs for a broad array of nutritional analyses, including major and trace minerals (including iron, copper, zinc, manganese and selenium), major amino acids (including methionine and cysteine), fat-soluble vitamins (vitamins A, D, E), B-vitamins (thiamin, riboflavin, niacin, B-6), vitamin C, and major fatty acids (including long-chain polyunsaturated fatty acids), with blind submission of NIST reference materials as a crosscheck of analytical accuracy.

To evaluate specific diets of individual otters, we will also need to sample additional prey species and prey size classes (over and above the main prey items mentioned above) so that as many prey types as possible can be represented. We will strive to analyze all prey species that represent more than 5% of the identifiable items in any individual otter diet. We anticipate that this may amount to 15-20 prey species, including the 10 major prey items. These additional prey samples will be obtained at the same time as collections of major prey, on an opportunistic basis. Laboratory analyses for these additional samples will be performed at SNZP. The nutrient composition of each diet will be calculated on a mass basis, multiplying the fractional proportion of total diet mass from each prey type by the nutrient composition of this prey type, and adding all such fractions together to determine overall dietary nutrient composition. We will examine correlations between nutritional composition, diet cluster, animal sex and age, reproductive status, health profiles and morbidity.

Three additional diet types from outside the Sanctuary will also be evaluated for comparison: that of sea otters that historically foraged in Elkhorn Slough, that of newly established sea otters foraging on San Nicolas Island, and that of sea otters foraging in inner Glacier Bay, Alaska.

Figures and Images

Figure 1. The giant-spined sea star, Pisaster giganteus, is consumed by sea otters. The Sea Otter Nutrition Project (SONP) lists this as the second-most abundant sea star encountered. The edible parts to a sea otter are found inside each of the five rays (arms), which contain the gonads and pyloric cecum (part of digestive tract). Otters suck these structures out then toss the star back into water. This is a popular food for pups during maternal foraging dives, although not necessarily a preferred food.

Figure 2. In Stillwater Cove, Monterey County, CA, divers Mike Kenner and Dr. Jim Estes prepare to leave the boat and search for urchins, abalone, and subtidal snails. June 2004.

Figure 3. After two dives, a large number of prey samples, covering a wide range of species, are collected and held in bins of sea water until they can be processed. June 2004.

Figure 4. A bat star (Patiria miniata, formerly Asterina) collected while diving in waters south of Estero Bay, San Luis Obispo County, CA, is spawning. June 2004.

Figure 5. Technician Alice Green sorting purple urchins (Strongylocentrotus purpuratus) collected from tidepools at Rancho Marino, San Luis Obispo County, CA, into size classes (juvenile, small, medium, large). September 2004.

Figure 6. Katherine Mertes (research intern) measures fat in sea otter prey by Soxhlet ether extractions at the Nutrition Lab. August 2004.


  • Oftedal et al. (2007)
    Nutritional constraints on the southern sea otter in the Monterey Bay National Marine Sanctuary, and a comparison to sea otter populations at San Nicolas Island, California and Glacier Bay, Alaska.
    11.9 MB PDF